Lyophilization or freeze drying is an important downstream process for stabilizing pharmaceutical compounds. The control and repeatability of lyophilization cycles are crucial for achieving consistently high product quality. Although the obvious parameters of shelf temperature and chamber pressure may be well controlled, the lack of control of the ice nucleation temperature (the temperature at which the product freezes) can adversely affect product uniformity and lead to suboptimal freeze-drying cycles. This study describes a novel means to control ice nucleation using a sterile cryogenic ice fog that is applicable to laboratory-, pilot-, and production-scale lyophilizers. Test results demonstrate the scalability and robustness of this technique.

Lyophilization or freeze drying is an important downstream process for stabilizing pharmaceutical compounds. It involves removing water and solvents from a product by sublimation and desorption to levels that will not support biological or chemical reaction. It is an excellent method to extend the shelf life of sensitive compounds for storage and transportation without subjecting them to detrimental high temperatures, and the only method available for a majority of biological compounds. Consequently, lyophilization continues to be indispensible to the pharmaceutical industry, despite its high cost and complexity.

Lyophilization consists of two major steps: freezing solutions, and drying the frozen solid under vacuum through sublimation and desorption. The drying step is divided into two phases: primary drying (i.e., ice sublimation) and secondary drying (i.e., liquid desorption). A successful lyophilization cycle can be defined by dried product that is visually and functionally acceptable with a short reconstitution time, potent active ingredients, and increased shelf life. The control and repeatability of the cycle are crucial for achieving consistently good product quality. Demand for lyophilization technology is growing because of the high value of the drugs being lyophilized as well as FDA initiatives such as quality by design (QbD) and process analytical technolgoy (PAT). Consequently, the industry has been quick to develop and adopt technologies that facilitate improved control of key process parameters. Controlling ice nucleation during the freezing cycle of lyophilization is one such tactic that is currently under investigation as a means to achieve more robust and scalable lyophilization cycles.

The importance of ice nucleation temperature

The onset of freezing, or ice nucleation, is one of the most important steps in the lyophilization cycle. For nonaseptic systems a particle or impurity often serves as the nucleation point that allows ice crystals to grow and the product to freeze. However, in aseptic systems of high purity the product sometimes cools below its freezing temperature without ice crystal formation because no particulates are available for ice nucleation, a process known as supercooling. Substances that cool below the freezing temperature without becoming solid are referred to as supercooled. The degree of supercooling determines the ice crystal structure, which in turn characterizes product resistance to water vapor flow during the drying cycle. Increased supercooling has been shown to form smaller, more numerous ice crystals, thus resulting in higher product resistance and increased drying times. Studies have shown a 1–3% increase in primary drying time for every 1 °C decrease in ice nucleation temperature (1, 2). Supercooling of vials during freezing can thus increase cycle times and operating costs.

Lack of uniformity in ice nucleation temperature caused by vial supercooling can lead to vial-to-vial variability in ice crystal structure. Vials that freeze at high temperatures dry faster than those that freeze at low temperatures, making it difficult to have a drying cycle that is optimal for all vials. This variability causes problems such as vial breakage and melt-back, and decreases overall yield and product uniformity.

In addition, variability in ice nucleation increases the uncertainty in scaling up a cycle from laboratory (nonaseptic) to production scale (aseptic). A cycle optimized at lab scale may have entirely different drying time requirements at production scale due to the higher degree of supercooling expected in particulate-free, production-grade environments. Variability in ice nucleation is compounded by vial-to-vial variations in drying behavior due to variable ice structure.

Methods to address issues related to nonuniform ice nucleation

Although ice nucleation is an important parameter for achieving robust cycles, there have been very few attempts to achieve it at commercial scale until recently. The standard practice has been to use an annealing cycle, which involves raising the product temperature after freezing to a temperature above glass transition, and then holding. This method results in the formation of larger ice crystals at the expense of smaller ones, and helps minimize the variability in drying behavior. However, annealing may not be well tolerated by protein systems that are susceptible to denaturation. In addition, the benefits of shorter drying times may be offset by the additional time required for the annealing cycle. Lastly, annealing fails to address the root cause of variable ice structure, which is the lack of a uniform ice nucleation temperature, and can only help to repair the damage already caused.

Other methods that have been tried at laboratory scale include using nucleating agents such as silver iodide and bacteria, ultrasonic vibration of the product, etched vials, sudden depressurization, and ice fog. This study will focus on the last method, ice fog, and show its successful transition from a laboratory concept to a commercially viable technique.